Encounter With The Giant

In the final week of January 2000, project scientists congregated at JPL to further refine plans for the Jovian encounter and the Titan fly-bys. The first workshop was held to plan the fly-bys of Saturn's icy satellites, and planning began for a workshop that would be held at Oxford University in England to maximise synergy between the Cassini and Huygens teams.

In February, Cassini and Galileo undertook a Conjunction Experiment to observe the Jovian radio emissions stereoscopically, as Cassini had done with the Wind spacecraft during the Earth fly-by. Such observations could be made only when the geometry of the two platforms was favourable in relation to the planet. Another opportunity would present itself in May.

The Orbiter Science Operations Working Team met in March to define the plan for the 'Jupiter Subphase' of the interplanetary cruise, the Spacecraft Office started to refine the Saturn Orbit Insertion Critical Sequence, the Satellite Orbiter Science Team integrated the Iapetus observations into the orbital tour, and the Titan Orbiter Science Team worked out precisely how the radar would achieve a minimum of 25 per cent high-resolution coverage of Titan's surface. In early April, the Probe Relay Critical Sequence Team met to consider the results of the end-to-end test that had been made in February. No one seriously expected Cassini to be disabled in passing through the asteroid belt, but a collective sigh of relief was expressed when it emerged in mid April. Despite the potential hazard, NASA's record was now 7 for 7: because after Pioneer 10 had blazed the trail, its mate had followed it through, as had the two Voyagers and Ulysses and Galileo. ''It's pretty routine,'' said Robert Mitchell. ''There's a lot of material in the belt, but there's also an awful lot of space out there.'' A week later, Cassini crossed the orbit of Comet Wild 2. Meteor streams may be associated with the orbits of comets, but the Cosmic Dust Analyser did not report any hits. A full-scale model of the Huygens probe was the centrepiece of a two-day meeting of the European Geophysical Society's XXV General Assembly in Nice, France, in late April, at which more preliminary results from the Venus and Earth fly-bys were presented. The Journal of Geophysical Research agreed to publish the formal papers in a special issue entitled 'First Results from Cassini'. With Cassini nearing superior conjunction on the far side of the Sun, it was buffeted on 9 May by the shockwave from a 'coronal mass ejection'. Meanwhile, the planners agreed on Enceladus fly-by option E3 as the 'baseline' orbital tour, and scheduled a review in November 2006 to consider switching to the E3a option. The Satellite Orbiter Science Team continued to work on the allocation of time and resources for studying the icy satellites, in this case integrating the Dione fly-by. Consideration was given to the observations to be made during the Saturn Orbit Insertion period which, although a critical phase of the mission, would facilitate very-high-resolution imaging of the ring system and offer a never-to-be-repeated opportunity to sample the inner magnetosphere.

On 14 June, Cassini's main engine was fired for 5.8 seconds for a 0.6-metre-per-second trajectory correction manoeuvre. Voyager 2's imagery of Phoebe from a range of 2.2 million kilometres had shown that there are intriguing bright patches on its rather dark surface, but the resolution had been poor. Cassini is to inspect Phoebe from a much closer range on its way into the system, and this manoeuvre was to refine this encounter.

After the successful in-flight reprogramming of the Galileo spacecraft in order to overcome a variety of faults, it had been decided that the Cassini mission would adopt a strategy of launching the spacecraft with a core of basic software and develop and transmit upgrades when they were required. Referring to this 'just-in-time' process, Earl Maize, the manager of the Spacecraft Operations Office, said: ''We get the right stuff done at the right time.'' In 1999, it had been decided to undertake a science campaign while passing through the Jovian system. In March and April 2000, software was uploaded to the Attitude and Articulation Control Subsystem to enable Cassini to orient itself using its system of reaction wheels, instead of firing its thrusters. In addition to saving propellant, this would facilitate much sharper imaging. At the end of July, the Command and Data Subsystem was upgraded to improve the processing of large amounts of scientific data and permit simultaneous use of the two solid-state data recorders. ''The studies of Jupiter are a rehearsal of the process of planning and executing complex sequences of operations to share the spacecraft's data-handling capabilities among the various instruments,'' reflected Brian Paczkowski, the science planning manager. Having two spacecraft offered a unique opportunity to study the interaction between the solar wind and the Jovian magnetosphere. Initially, Cassini would monitor upwind and the Galileo spacecraft would simultaneously report from its vantage point inside the magnetosphere. A few weeks later, as Cassini penetrated the magnetopause, Galileo would temporarily re-emerge into the solar environment at apojove. ''Having two spacecraft there at once is, possibly, the only chance in our lifetime to simultaneously relate changes in the solar wind to conditions inside Jupiter's giant magnetosphere,''

pointed out Scott Bolton, who was on the teams for both Galileo's Plasma Wave Spectrometer and Cassini's Radio and Plasma Wave Spectrometer.

Meanwhile, in mid-August, the latest observations of Titan were reported at the General Assembly of the International Astronomical Union held in Manchester, England, confirming the fascinating nature of the intended landing site for the Huygens probe. At JPL, the Galileo scientists presented the latest observations of the Jovian moons to their Cassini colleagues in order to assist them in planning their own programme. On 25 August the Jupiter Readiness Review sought to resolve outstanding activities. In early September, with Jupiter looming, Cassini encountered submicron dust from Io's volcanic plumes which had been accelerated by Jupiter's magnetic field and sent into interplanetary space in the form of a collimated stream. On one day, the Cosmic Dust Analyser counted more than 250 hits, but this was fairly mild for such streams because the Galileo spacecraft's detector had been reporting 20,000 hits per day by this point in its approach to the planet.

Later in the month, in the never-ending series of meetings to review progress and to plan future activities, the Spacecraft Operations Office held its preliminary design review for the Probe Relay Sequence. As the Cassini and Huygens teams pored over the detailed information on how the relay was to occur, and correlated this with the end-to-end test of the system conducted earlier in the year, it became apparent that there was a serious flaw in the design which meant that not all of the data transmitted by the probe would be received by Cassini. In effect, when the technical specifications for the relay link had been agreed by the two teams, no allowance had been made for the fact that Cassini, inbound on its elliptical capture orbit, would be racing towards Titan so rapidly that the Doppler effect would shift the probe's frequency so far that the signal-to-noise ratio in the receiver's pre-tuned narrow bandwidth would degrade so much (some 10 decibels) as to render it unreadable. Such a frequency shift had been designed into the relay for the Galileo probe, but Cassini and Huygens had been designed by different teams and this subtlety had been omitted from the specifications - a fact that had gone unnoticed in the design review process.74 It came to light only when the Deep Space Network's engineers, in simulating the transmission from the probe, realised that there was no compensation. Fortunately, the fault had been detected early enough for a 'workaround' to be devised so that the Huygens probe's science potential could be fully realised.

Meanwhile, on 1 October, Cassini activated its Imaging Science Subsystem and recorded its first view of Jupiter. Even at a range of 84 million kilometres, the clarity of the image was astonishing. ''This spacecraft is steadier than any I have ever seen,'' enthused Porco, the leader of the team. ''It's so steady the images are unexpectedly sharp and clear, even in the longest exposures taken in the most challenging spectral regions.'' The exceptional resolution was a tantalising taste of the system's scientific potential. Cassini showed Jupiter's cloud tops in exquisite detail. The narrow-angle camera's near-infrared imagery from 168 planetary rotations through to 9 December was sequenced into a movie.75 Analysis of this unprecedented data set produced a few surprises. ''This is the first movie ever made of the motions of clouds near Jupiter's poles,'' noted Porco, ''and it seems to indicate that one notion concerning the nature of the circulation on Jupiter is incomplete at best, and possibly wrong.'' A

A view of Jupiter taken by Cassini on 7 December 2000. Note Europa's shadow on the disk (the moon itself is out of frame).

popular model posited that the alternating bands of east-west winds are the exposed edges of deeper, closely packed rotating cylinders extending north-south through the planet.76 At the planet's surface, one would see only east and west winds, alternating with latitude symmetrically about the equator. The fly in the ointment was that the winds in the polar regions in the movie did not behave in this way. Perhaps Jupiter's wind pattern involves a mix of cylindrical structures near the equator and some other mechanism near the poles.

At first sight, the mottling in the polar regions appeared to be chaotic, but in fact this is not the case. Thousands of spots, each an active storm system larger than the largest terrestrial storm, were seen jostling one another as they streamed together in any given latitudinal band; only a few changed bands. ''Until now, we didn't know the lifetime of these storms,'' pointed out Andrew Ingersoll of Caltech. Although some spots merged with one another, most persisted throughout the entire sequence. ''The smaller and more numerous storms at high latitude share many of the properties of their larger cousins, like the Great Red Spot, at lower latitudes,'' noted Ingersoll. Why the storms last so long is a mystery of Jupiter's weather. Storms on Earth last approximately seven days before they break up and are replaced by other storms. The new data heightens the mystery, because it shows long-lived storms at the highest latitudes where the weather patterns are more disorganised than at lower

On 12 December 2000, Cassini snapped Io in transit across Jupiter's disk, casting its shadow on the planet to the east of the Great Red Spot.

latitudes. ''Perhaps we should turn the question around, and ask why the storms on Earth are so short lived,'' Ingersoll mused. As often happened, in observing another planet, we gained insight into our own world. ''We have the most unpredictable weather in the Solar System, and we don't know why.''

Cassini's Composite Infrared Spectrometer measurements of the abundance and distribution of various gases in the Jovian atmosphere (including methane, acetylene, benzene and other hydrocarbons, water and carbon dioxide) would provide insight into the photochemical processes at work in the stratosphere. Galileo's radiometer was sensitive into the far-infrared, but it had not been able to chart the atmosphere with the spatial and spectral resolution provided by the CIRS.77 The temperature fields of the stratosphere and the tropopause that forms its lower boundary would provide a measure of the zonal winds and the thermal anomalies resulting from 'atmospheric waves' with lifetimes ranging from hours to months.78

Although by this time the Galileo spacecraft had been orbiting Jupiter for five years, its communications capacity was limited by the fact that its high-gain antenna had not deployed properly and it had been unable to yield such intensive synoptic coverage. Cassini's fly-by was a welcome opportunity to ameliorate this aspect of its predecessor's mission. About 60 years ago, an atmospheric disturbance south of the Great Red Spot had given rise to three 'white ovals'. Two of these had merged in 1998, but Jupiter had been at superior conjunction at the time and Galileo had not been able to monitor their merger, but by a lucky fluke of timing Cassini was able to document the final merger.79

The Ultraviolet Imaging Spectrograph was providing an excellent combination of spatial, spectral, and temporal resolution data on Io's plasma torus. This first-ever imaging spectroscopy of the torus took the form of multiple overlapping exposures, each at a different emission wavelength. ''We can see the entire donut of glowing gas in all its invisible colours,'' pointed out L.W. Esposito. In mid-November, the Visual and Infrared Mapping Spectrometer made its first observations of the atmosphere.80 Between 18 and 23 November, Cassini was immersed in the shock wave from yet another coronal mass ejection. ''Such major disturbances in the solar wind may well cause Jupiter's magnetosphere to flap around significantly,'' said Andrew Coates of the Mullard Space Science Laboratory, part of London University, leading the CAPS instrument's electron spectrometer team. In fact, as it dived back towards Jupiter from apojove, Galileo had no sooner entered the magnetosphere than the pressure of the solar wind so compressed the magnetosphere that the spacecraft was once again in the solar environment. As Cassini was to make a fairly distant Jovian fly-by, it was possible that it would remain outside the magnetosphere if the solar wind was very powerful. By the start of December, Cassini was within 30 million kilometres of Jupiter and was being accelerated by its tremendous gravitational attraction. Starting on 14 December, as Cassini and Galileo made particles and fields measurements, the Hubble Space Telescope, orbiting the Earth, monitored the Jovian auroral displays. The main stimulus for auroral activity is Io's presence deep in the magnetosphere, but the objective was to study the degree to which the magnetosphere's response to the gusty solar wind influenced the aurorae. "We know that the solar wind controls the terrestrial aurora,'' explained J.T. Clarke of the Department of Atmospheric, Oceanic and Space Sciences at the University of Michigan, leading the team using the Space Telescope Imaging Spectrograph, "but we are not sure how they influence the aurora on Jupiter.'' The insight thereby achieved would be able to be applied to the study of extra-solar planets, because most of the cases identified to date are 'Jupiter class' giants orbiting very close to their parent stars.

On 15 December, a fortnight from the fly-by, and while working autonomously, something prompted Cassini to deactivate its reaction wheel system and revert to using thrusters for attitude control. It continued its observational programme in this mode. The problem was not noticed until the routine telemetry downloading session on 17 December, at which time the engineers saw that one of the wheels had become sluggish. If the motor was commanded, the wheel took 5 to 10 times the nominal amount of force to act, therefore the system was ordered off again to enable the fault to be analysed. Although Cassini could have continued science operations employing its thrusters for attitude control, Robert Mitchell chose to save the propellant to avoid eroding the healthy margin that had been carefully built up for the primary mission, so on 19 December planned activities that would require the spacecraft to point in a specific direction were cancelled. ''We're responding cautiously while we test the systems,'' Mitchell noted. The particles and fields measurements of the spacecraft's environment would continue unabated. Cassini adopted an orientation with its high-gain dish antenna pointed towards the Earth in order to maintain communications. On 18 December, Cassini passed Himalia, the largest of a group of outer moonlets believed to be captured asteroids,81,82 at a range of 4.4 million kilometres. As Himalia had not been on Galileo's list, Cassini had been tasked to determine its size, rotation and composition. Although, at best, Himalia would span only 7 pixels and the resolution would be 25 kilometres per pixel, it would be a useful observation.83,84 Himalia has the overwhelming majority of the group's mass, which suggested that the others (Leda, Lysithea and Elara) had been 'chipped' from it. Although several images were taken, showing that the side of Himalia that faced the spacecraft was about 160 kilometres across, the plan to take a series of images to determine its rotational period was frustrated by the decision to minimise manoeuvring. A recent study exploiting sophisticated software to identify moving objects had turned up a surprisingly large number of small moons orbiting far from Saturn,85 so imaging Himalia was a rehearsal for future work, as even basic

Cassini's route through the Jovian system took it within 4.4 million kilometres of Himalia, one of the small outer moons. Despite suffering an attitude-control problem, the spacecraft turned to record this unique view using a near-infrared filter. The inset shows the moon enlarged by a factor of 10 and a graphic of the illumination from the left. The resolution is about 27 kilometres per pixel. The dimension of the visible part is roughly 160 kilometres in the north-south direction, and in all likelihood it is non-spherical.

Cassini's route through the Jovian system took it within 4.4 million kilometres of Himalia, one of the small outer moons. Despite suffering an attitude-control problem, the spacecraft turned to record this unique view using a near-infrared filter. The inset shows the moon enlarged by a factor of 10 and a graphic of the illumination from the left. The resolution is about 27 kilometres per pixel. The dimension of the visible part is roughly 160 kilometres in the north-south direction, and in all likelihood it is non-spherical.

knowledge of the physical parameters of these moons will provide useful statistical information on the kind of bodies that are captured by giant planets.

Over the next few days, the engineers subjected the sluggish reaction wheel to a programme of tests. At first it continued to misbehave, but then, several days later, resumed normal operation. It was concluded that an irregular distribution of the lubricant in the motor might have caused the problem. ''That's our leading theory, but we may never know for sure,'' Mitchell admitted. The wheels were commanded back on-line on 21 December, and monitored for a week while they maintained the Earth-pointing attitude. ''Everything has been working smoothly,'' reported Mitchell after the trial, ''so we'll resume all scientific observations.'' A few hours later, the spacecraft slewed around to aim its remote-sensing instruments towards Jupiter. On 29 December, the Galileo spacecraft continued its exploration of the Jovian system by making a fly-by of Ganymede while the moon was in Jupiter's shadow, providing an opportunity to measure how it cooled down in eclipse to determine the thermal characteristics of the non-ice components of its surface. While close to Jupiter, Cassini and Galileo made a coordinated study of how dust from Io's plumes is accelerated in the magnetosphere until it escapes the giant planet's gravitation.86 Both vehicles carried dust detectors supplied by the Max Planck Institute for Astrophysics. On 29 December, a stream initially noticed by Galileo was seen 9 hours later by Cassini. The manner in which a stream swept over first one spacecraft and then the other was another way in which Cassini's passage was able to enhance Galileo's mission. When Cassini was launched in 1997, few had dared to hope that as it flew past Jupiter, Galileo, having survived triple its expected total radiation exposure, would still be essentially fully functional.

At 10:12 UTC on 30 December Cassini passed within 10 million kilometres of Jupiter. Although at a planetocentric distance of 135 radii, the slingshot nevertheless accelerated the spacecraft by 2,218 metres per second and placed it onto a trajectory that would result in an encounter with Saturn.

Early on 28 December, Cassini had encountered Jupiter's magnetosheath. This indicated that the magnetosphere had inflated once again in a lull in the solar wind. Although the particles and fields instruments reported numerous encounters with the bow shock between then and 3 January, the distant fly-by did not penetrate the magnetosphere. Coordinated observations with Galileo continued as Cassini flew on. ''We're making the most comprehensive characterisation of the radio emissions from Jupiter ever,'' assured William Kurth of the University of Iowa and a member of the Radio and Plasma Wave Spectrometer team.87'88'89'90 The turbulent bow shocks were also noted by Cassini's Plasma Spectrometer91 and magnetometer.92 The Ultraviolet Imaging Spectrograph93,94 and the Hubble Space Telescope's Imaging Spectrograph monitored auroral activity at the time of closest approach.95 ''I've been observing Jupiter's aurora for 22 years and these images have provided an enormous amount of new and interesting data,'' said J.T. Clarke, leading the Hubble team. Although Io is primarily responsible for the Jovian aurorae, Cassini's Plasma Spectrometer showed that variations in the solar wind appeared to be correlated with fast fluctuations in brightness, known as 'polar cap flares'. ''We collected more data in two weeks than we've amassed in several years,'' noted Hunter Waite, leading the Ion Neutral Mass Spectrometer team. ''This will help us determine if our theories of how the aurorae behave are right.'' Beyond Jupiter, Cassini was able to observe the dark hemisphere, as the Hubble Space Telescope continued to monitor the dayside. ''It's a rare chance to view Jupiter from two vantage points simultaneously. As Jupiter rotates and the solar wind changes, we can collect images and solar wind data without interruption. That is something we have never been able to do before,'' explained Waite.

The trajectory of the Galileo spacecraft during Cassini's Jovian fly-by.

Although Cassini was able to observe Jupiter's four large moons only from afar, its Composite Infrared Spectrometer was sensitive to a wider wavelength range than Galileo's Near-Infrared Mapping Spectrometer, with improved spectral resolution, so despite the inherent trade-off of spatial resolution it was able to make useful observations of the composition and thermal structure of their surfaces. However, even at the closest point of approach, none of the satellites spanned more than a pixel to the Visual and Infrared Mapping Spectrometer.96 In monitoring Io's volcanoes, Cassini saw plumes rising hundreds of kilometres above Pele and Tvashtar.97 By chance, in November 1999 Galileo had seen Tvashtar spewing a fire fountain of lava some 2 kilometres into the sky from a curvilinear fissure. An eruption from a much longer fissure nearby was spotted by terrestrial observer Frank Marchis on 16 December 2000.98 Remote imaging by Galileo shortly afterwards revealed that the plume from this new eruption had deposited a large ring of reddish pyroclastic similar to the halo that has adorned Pele since at least the time of the Voyager fly-bys. Although remote, Cassini's imagery of Io while the moon was in Jupiter's shadow was welcome because thermal emissions from 'hot spots' can be studied without the solar reflection that is present when the surface is illuminated.99 In the case of Io, the faint auroral glows in the moon's tenuous volcanically-generated atmosphere that are induced by interactions with the charged particles flowing in the flux tubes could also be observed when in eclipse.100'101'102 The Ultraviolet Imaging Spectrograph's imagery was sequenced into a movie that depicted Io's plasma torus gyrating in the extreme ultraviolet. ''We're visualising the torus, and seeing it evolve and change in a level of detail that people have never seen before,'' said team leader L.W Esposito. Cassini also investigated the inner moons Metis and Adrastea, which the Galileo results had suggested were the source of the fine dust in the rings. Once across the planet's orbit, Cassini turned to enable the CIRS to observe the forward-scattered sunlight from the rings, and to employ filters to determine their chemical composition, in one case seeking the spectral signature of aluminosilicates. In effect, this was a rehearsal for the observations it would make of Saturn's rings.

The Magnetospheric Imaging Instrument detected a cloud of neutral atoms and ions pervading the interplanetary medium at least 25 million kilometres from the Jovian system. INCA images taken near closest approach were sequenced to yield a large-scale view of the compression and expansion of the magnetosphere in response to gusts in the solar wind.103 By making coordinated observations the MIMI sensors were able not only to monitor fluctuations in the magnetosphere's shape, but also to map its chemical composition. ''CHEMS was able to show that a significant portion of the particles in the cloud were sulphur and oxygen, with sulphur dioxide probably present as well,'' pointed out D.C. Hamilton, leader of this team. ''Sulphur dioxide is the main gas emitted by volcanoes, indicating Io as the likely origin for much of the gas cloud.'' Once neutral atoms from the plumes are ionised, they are 'picked up' by the rapidly rotating magnetosphere. Although the ions are accelerated sufficiently to escape the planet's gravitational field, their electrical charges enable the magnetic field to retain them. However, free electrons also circulate in the magnetosphere, and if an electron neutralises an energetic ion it can escape. Once in interplanetary space,

Cassini is the first spacecraft able to image the bubble of charged particles trapped within a planet's magnetic field. The Magnetospheric Imaging Instrument's Ion and Neutral Camera captured this view of the Jovian magnetosphere in early January 2001. For perspective, the planet's disk, Io's plasma torus, and a series of magnetic field lines are superimposed. (Courtesy of the Applied Physics Laboratory of Johns Hopkins University.)

Cassini is the first spacecraft able to image the bubble of charged particles trapped within a planet's magnetic field. The Magnetospheric Imaging Instrument's Ion and Neutral Camera captured this view of the Jovian magnetosphere in early January 2001. For perspective, the planet's disk, Io's plasma torus, and a series of magnetic field lines are superimposed. (Courtesy of the Applied Physics Laboratory of Johns Hopkins University.)

many atoms are re-ionised, this time by solar ultraviolet, and CHEMS was able to detect them.104,105 ''This fly-by has been an excellent test of MIMI's capabilities,'' Hamilton reported, ''and it has allowed us to make important refinements to the software.''

As Cassini moved beyond Jupiter, it was able to study the magnetotail. It spent several days in the magnetosheath until noon on 9 January, when it slipped through the magnetopause, but this washed back and forth several times later in the day. The spacecraft did not finally enter the magnetosphere until the following day, by which time it was 14 million kilometres downwind. Galileo, half as far from the planet and trailing behind it, was also inside the magnetosphere when the solar wind gusted and the magnetosphere rapidly shrank, leaving both spacecraft in the solar environment. Cassini spent most of January and February skating along the magnetosphere's dusk flank. A 0.5-metre-per-second manoeuvre on 28 February served both to correct the small dispersions resulting from the fly-by and to perform preventative maintenance on the bi-propellant system's main engine, whose specifications require it to be fired for at least 5 seconds in every 400 days in order to clear any oxidation build-up. The post-encounter phase of the Jovian studies continued until early May 2001. Cassini spent much of this period with its high-gain antenna aimed at Jupiter. This phase of activity, which was conducted in concert with several terrestrial antennas, had two objectives. Firstly, Cassini's proximity to Jupiter meant that the planet's disk filled the antenna's narrow beam, and the thermal emission from the atmosphere offered a welcome opportunity to calibrate the radar's radiometer. Simultaneous multi-band observations by the Deep Space Network antennas provided a further check. Noting the radiometer's extreme sensitivity, the team that was planning the primary mission decided to accept 'targets of opportunity' such as Saturn's disk. Of more immediate interest to the Jovian magnetosphere specialists was that when in 'listen-only' mode Cassini could detect the synchrotron emission from the relativistic electrons trapped in the radiation belts concentrated above the equator, and the radar's data-processing system provided imagery. Such emissions have been studied at various frequencies by radio telescopes since the 1960s. Cassini was able to map the radiation belts emitting at a wavelength of 2.2 centimetres, which is denied to terrestrial telescopes because the thermal output from the planet swamps the synchrotron emission. Being closer, Cassini was able to differentiate between the synchrotron emission from the radiation belts in space and the emissions from the atmo-sphere.106'107'108 This was done by nodding the spacecraft back and forth in order to scan across the 'target' several times, then rolling 90 degrees to repeat the procedure. The synchrotron radiation could be identified by its characteristic polarisation. The Very Large Array in New Mexico and the Goldstone-Apple Valley Radio Telescope in California made simultaneous observations at 20 and 90 centimetres.

''Cassini has been able to 'anchor' the high-energy end of the electron spectrum from Jupiter's radiation belts for the first time,'' said S.J. Bolton on 28 March 2001, when the preliminary analysis was presented to the European Geophysical Society in Nice, France. The various data sets were integrated to chart the energetic particle distribution within Jupiter's radiation belts. ''We got some surprises,'' Bolton added. In particular, the highest-energy electrons were less populous than predicted.109 As it turned out, the magnetosphere is asymmetric. ''The dusk flank of the magnetosphere is a surprising contrast to the dawn flank,'' S.M. Krimigis explained at the American Geophysical Union in Boston in May 2001. This solved the mystery. The lopsided magnetosphere is leaky. There is an unexpected abundance of high-energy particles bleeding out of one side. These escaping electrons and ions might be riding magnetic field lines that are attached to the planet at one end and are waggling freely on the other, unlike the field lines closer to the planet which loop between its northern and southern magnetic poles. There was a dearth of the highest-energy electrons because they were leaking away.110

In retrospect, the most astonishing aspect of the Jovian encounter is that when Cassini was launched, budgetary constraints had imposed a flight plan in which the spacecraft would have made the fly-by with most of its instruments switched off!

Carolyn Porco was delighted with the Imaging Science Subsystem. ''The camera has performed beyond our wildest imaginings - and that's saying something, because we've been imagining this for a decade now.'' And team member Carl Murray of the University of London was looking forward to Saturn, ''I'm confident that... the best is yet to come.''

After a detailed study of the sluggishness suffered by one of the reaction wheels, it was decided that, on concluding its post-Jovian sequence in July, Cassini should minimise the use of the wheels and employ its thrusters for attitude control on the long cruise to Saturn.

When Cassini's high-gain antenna was employed as a radiometer, it could 'see' the radio emission from high-energy electrons in Jupiter's radiation belt. Since the magnetic field is inclined to the planet's rotational axis, the structure precesses.

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